The present invention relates to electrolytic graining, particularly the electrolytic graining of aluminum sheets so as to provide the sheet with a surface suitable for use as part of a lithographic printing plate.
It has long been known to be advantageous to form a printing plate by coating a lithographically suitable photosensitive composition on to the surface of an aluminum sheet substrate for subsequent exposure to light through a mask with eventual development. The oleophilic image areas which remain accept and transfer ink during the printing process and the hydrophilic non-image areas accept water or aqueous solutions during printing to repel such greasy inks.
It has long been known that if the surface of the aluminum substrate were grained, either mechanically, for example by use of wire brushes or particulate slurries, or electrochemically by use of electrolytic solutions of acids such as nitric acid that the printing life of a plate may be substantially extended.
Electrolytic graining of aluminum and the electrolytic process has many advantages over mechanical graining. (See, for example, U.S. Pat. Nos. 3,072,546 and 3,073,765). For certain applications, a very fine and even grain is desired. For example, when the aluminum is to be used as a support for lithographic printing plates such characteristics are especially advantageous. A fine and even grain can be obtained in an electrolyte consisting of an aqueous solution of hydrochloric acid but the current density employed must be kept quite low or pitting of the aluminum surface will take place and, as a result of the low current density, it requires a relatively long period to complete the graining.
French Pat. No. 2,110,257 describes a process for electrolytic graining of aluminum in which the graining is carried out at a current density of 0.5 to 10 A/dm.sup.2 (amperes per square decimeter) in an aqueous electrolyte solution containing 0.5 to 2 percent by weight of hydrochloric acid and 0.1 to 1.5 percent by weight of boric acid. This process provides a fine and even grain but it is relatively slow with the time required for graining typically being about 5 minutes, or longer. According to this patent, the use of concentrations of hydrochloric acid or boric acid above 2 percent, or the use of a current density exceeding 10 A/dm.sup.2, yields a coarse and irregular surface that is not suitable for use in lithographic printing.